THE FINDINGS Kanazawa used data from the National Longitudinal Study of Adolescent Health, a database of 20,745 adolescents that includes information about their families, friends, schools, and communities. In 2001, when the kids were 18–28 years old, 2,972 of them had their own children. The interviewers judged the desirability of each of these subjects, now parents, on a scale of 1 (very unattractive) to 5 (very attractive). "About average" got a 3. (Just like in Lake Wobegon, nearly everyone was above average.)

THE PROBLEM Since being beautiful tends to pay off for women more than for men, evolutionary psychologist Satoshi Kanazawa of the London School of Economics wondered if natural selection might influence the sex of the offspring of beautiful parents. Assuming they can pass along their good looks, are the Brangelinas of the world more likely to have girls than boys?

Kanazawa found that 56 percent of the most beautiful people in the survey had a daughter first compared with only 48 percent of the merely attractive, average, homely, and downright ugly people combined. That sounds weird, but a fifty-fifty sex ratio is not always the norm. In species like red deer, spider monkeys, and Venezuelan opossums, environmental conditions strongly influence the sex mix of the offspring. Consider what happens to opossums in hard times: Since runty males might not find mates but skinny females can usually get a partner, more daughters are born.

While Kanazawa's methods seem rather subjective, it is theoretically possible that—if physical attractiveness really does increase the reproductive success of daughters more than sons—natural selection could find a way to make better-looking people more likely to have daughters. Potential parents will always run up against uncertainty, however. "I can't predict the sex of any one child," says Kanazawa. "I can't say that if you're tall and geeky and beautiful then it's two against one and boy wins."

THE RESEARCHER Kanazawa says he doesn't want to be sued for this advice, but "by marrying a beautiful spouse you are slightly increasing the chance that you'll have a daughter." He has also published papers claiming that big people, violent men, and engineers and mathematicians have more sons, while nurses and schoolteachers have more daughters. How could that possibly work? "How is a question I don't ask; I only ask why," says Kanazawa.